Heat-shrinkable multilayer packaging film comprising inner layer comprising a polyester

ABSTRACT

A heat-shrinkable multilayer film comprises (A) a first layer, which is an outer layer, and which comprises polyolefin; (B) a second layer comprising at least one member selected from the group consisting of polyolefin, polystyrene, and polyurethane; (C) a third layer comprising at least one member selected from the group consisting of amorphous polyester and polyester having a melting point of from about 130° C. to about 260° C.; (D) a fourth layer, which is an outer layer, the fourth layer comprising at least one member selected from the group consisting of polyester, polyamide and polyurethane. The first layer preferably serves as a seal layer in a heat-shrinkable bag. The third layer provides enhanced impact strength, optics, grease-resistance, and free-shrink of the film, and renders the tape more easily orientable. The high melting polyester, polyamide, and/or polyurethane of the fourth layer permits at least two bags, having product therein, to be stacked on top of one another and sealed simultaneously, without sticking to one another, thereby doubling the output of a vacuum chamber machine. A bag and a process of making a packaged product are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/034,836, filed Mar. 4, 1998, now U.S. Pat. No. 6,610,392.

FIELD OF THE INVENTION

The present invention relates to heat-shrinkable films, especiallymultilayer, heat-shrinkable films suitable for use in the packaging ofproducts. The present invention is also directed to a packaging methodusing such films.

BACKGROUND OF THE INVENTION

There is a need for multilayer heat-shrinkable films and articles ofmanufacture made therefrom, which have high impact strength, especiallyat elevated temperatures, high free shrink at 185° F., high modulus,high gloss and package presentation, good sealability and seal strength,and stack/overlap sealing capability, and which can be easily oriented.This combination of features is not currently available. There is also astrong desire in the marketplace for thin films which also possess theabove-described combination of features. The process of downgaugingfilms without sacrificing performance attributes not only utilizes lesspolymeric material (which is better for the environment) but also lowersthe cost to the end-user.

Recently it has been discovered that certain commercially-available bagscan be sealed when stacked on top of one another, i.e., without stickingto one another. This non-sticking characteristic provides an advantagefor packaging in a vacuum chamber, because the chamber, althoughtypically having only one sealing means, has more than enough spacetherewithin for multiple bagged products which are to be sealed afterevacuation of the atmosphere from the chamber. Thus, the non-stickingfeature enables the evacuation and sealing of more than one bag at atime in a vacuum chamber, thereby increasing the production rate of thevacuum chamber packaging apparatus.

U.S. Pat. No. 5,336,549, to Nishimoto et al., discloses aheat-shrinkable film which can be made into bags. Apparently, users ofthis film, which is commercially available, hive discovered that bagsmade from the film can be stacked on top of one another during sealing,without sticking to one another (i.e., the bags are “stack-sealable”).This enables the output of vacuum chamber packaging machinery to be, forexample, doubled, if two bags are stacked on top of one another andsimultaneously sealed.

The film disclosed in the '549 patent has an outer layer of a polyester,and an intermediate layer of a polyamide having a melting point ofhigher than 160° C. and lower than 210° C. Although Nishimoto et aldiscloses a large group of polyamides for use in an inner layer,together with various polyesters for use in an outer layer, Nishimotodoes not disclose the use of an inner layer comprising polyester.

SUMMARY OF TIE INVENTION

We have discovered films which can provide a combination of desirablecharacteristics: high impact strength, especially at elevatedtemperatures, high free shrink at 185° F., high modulus, high gloss andpackage presentation, good sealability and seal strength, andstack/overlap sealing capability. Moreover, in the process of makingthese films, the orientation step is not difficult. In addition, filmsin accordance with our invention can be made significantly thinner thanprior art polyolefinic films and still possess comparable or superiorperformance attributes.

Moreover, we have discovered that at least the film of Example 1, below,which is in accordance with the present invention, exhibits one or moreof the following advantages over heat-shrinkable films which arepredominantly polyolefinic: a higher impact strength at both roomtemperature and elevated temperature, superior optics (e.g., gloss andhaze), and, superior grease-resistance.

Furthermore, it has also been discovered that it is not difficult toorient a cast tape in the process of making the heat-shrinkable film inaccordance with our invention. Orientation can be carried out to a highdegree, versus prior art films which comprise an inner polyamide layerhaving a high melting point. The ease of orientability and widerorientation window provided by the films of this invention also resultin a stable orientation process. While the film of our invention canalso provide high total free shrink, the process of orientation at alower temperature can also enhance the free shrink of our multilayerfilm. Furthermore, the films of this invention can be made so that theyare relatively free of optical defects (such as die-lines), versus filmscomprising a polyamide with a high melting point, i.e., greater than160° C. (e.g., polyamide 6, polyamide 66, polyamide 6166, polyamide6.6/6.10, polyamide 12, etc.). The use of an inner layer comprisingpolyester and an outer layer comprising a polyester is especiallypreferred in the films of this invention. The use of a polyester is alsopreferred in the films of this invention because it is significantlyless expensive than a polyamide.

In the packaging of a relatively rigid product which is not distorted byforces produced by a shrinking film, it is generally desirable toprovide a heat-shrinkable packaging film with as high a free-shrink aspossible, in order to provide the “tightest” possible packaging over theproduct. In general, a tighter package provides a superior appearance,all other factors remaining the same. Our film has a relatively highfree shrink, thereby enabling improved product appearance over a filmhaving a lower free shrink.

As a first aspect, the present invention is directed to aheat-shrinkable multilayer film comprising: (A) a first layer, which isan outer layer, and which comprises polyolefin; (B) a second layercomprising at least one member selected from the group consisting ofpolyolefin, polystyrene, and polyurethane; (C) a third layer comprisingat least one member selected from the group consisting of amorphouspolyester and polyester having a melting point of from about 130° C. toabout 260° C.; and (D) a fourth layer, which is an outer layer, thefourth layer comprising at least one member selected from the groupconsisting of polyester, polyamide and polyurethane.

Preferably, the film has a total free shrink, at 185° F., of from about40 to about 170 percent; more preferably, from about 50 to about 150percent; more preferably, from about 60 to about 130 percent; morepreferably, from about 65 to about 110 percent; more preferably, fromabout 70 to about 100 percent; and, more preferably, from about 75 toabout 95 percent.

Preferably, the film has a thickness uniformity of at least 20 percent;more preferably, at least 30 percent; still more preferably, at least 40percent; yet still more preferably, at least 50 percent; even yet stillmore preferably, at least 60 percent; still more preferably, at least 70percent; still more preferably, at least 80 percent; and, still morepreferably, at least 85 percent.

Preferably, the third layer comprises an amorphous polyester.

The fourth layer comprises at least one member selected from the groupconsisting of amorphous polyester and polyester having a melting pointof from about 130° C. to about 260° C. If the fourth layer comprises apolyester having a melting point (i.e., a non-amorphous polyester),preferably this polyester has a melting point of from about 150° C. toabout 250° C.; even more preferably, from about 170° C. to about 250°C.; still more preferably, from about 180° C. to about 240° C.; stillmore preferably, from about 190° C. to about 240° C.; still morepreferably, from about 200° C. to about 240° C.; and yet still morepreferably, from about 210 to about 235° C. Preferably, the polyester inthe fourth layer comprises from about 70 to about 95 mole percentterephthalate mer units; more preferably, from about 80 to about 95 molepercent terephthalate mer units; still more preferably, from about 85 toabout 90 mole percent terephthalate mer units.

In yet another preferred embodiment, preferably the fourth layercomprises at least one member selected from the group consisting ofamorphous polyamide and/or polyamide having a melting point of fromabout 130° C. to about 260° C. If the fourth layer comprises a polyamidehaving a melting point (i.e., a non-amorphous polyamide), preferablythis polyamide has a melting point of from about 150° C. to about 260°C.; even more preferably, from about 170° C. to about 250° C.; stillmore preferably, from about 180° C. to about 240° C.; still morepreferably, from about 190° C. to about 240° C.; still more preferably,from about 200° C. to about 240° C.; and yet still more preferably, fromabout 210 to about 235° C.

Preferably, the film has a gloss of at least 50 percent, as measuredagainst the fourth layer by ASTM D2457 (hereby incorporated in itsentirety, by reference thereto); more preferably, the gloss is at leastabout 55 percent; more preferably, at least about 60 percent; morepreferably, at least about 65 percent; more preferably, at least about70 percent; and still more preferably, at least about 75%. Preferably,the film has a haze of no more than 10 percent, as measured by ASTM D1003 (hereby incorporated, in its entirety, by reference thereto); morepreferably, a haze of from about 0 to about 7 percent; still morepreferably, from about 0 to about 5 percent.

Preferably, the film has a total thickness of from about 0.5 to about 10mils; more preferably, from about 1 to about 5 mils; more preferably,from about 1.3 to about 4 mils; still more preferably, from about 1.5 toabout 3.5 mils; yet still more preferably, from about 1.8 to about 2.5mils.

Preferably, the film further comprises a fifth layer which serves as anO₂-barrier layer, the fifth layer comprising at least one memberselected from the group consisting of EVOH, PVDC, polyalkylenecarbonate, polyamide, and polyethylene naphthalate. Preferably, thefifth layer is between the third layer and the fourth layer.

Preferably, the film further comprises a sixth layer which comprises atleast one member selected from the group consisting of polyester andpolyamide, wherein the sixth layer is between the fourth layer and thefifth layer. More preferably, the film further comprises a seventh layerwhich is a tie layer, the seventh layer being between the second layerand the third layer, and an eighth layer which is also a tie layer, theeighth layer being between the fourth layer and the sixth layer.

Preferably, the first layer comprises ethylene/alpha-olefin copolymer,the second layer comprises ethylene/vinyl acetate copolymer, the thirdlayer comprises polyethylene terephthalate, the fourth layer comprisespolyethylene terephthalate, and the fifth layer comprises EVOH. Morepreferably, the first layer comprises a blend of homogeneousethylene/alpha-olefin copolymer and heterogeneous ethylene/alpha-olefincopolymer.

Preferably, the second layer is between the first layer and the thirdlayer, the third layer is between the second layer and the fifth layer,and the fifth layer is between the third layer and the fourth layer.

Preferably, the first layer has a thickness of from about 1 to about 60percent (more preferably, from about 10 to about 30 percent), based ontotal film thickness; the second layer has a thickness of from about 1to about 50 percent (more preferably, from about 5 to about 25 percent),based on total film thickness; the third layer has a thickness of fromabout 5 to about 40 percent (more preferably, from about 10 to about 25percent), based on total film thickness; the fourth layer has athickness of from about 1 to about 40 percent (more preferably, fromabout 4 to about 20 percent), based on total film thickness; the fifthlayer has a thickness of from about 1 to about 20 percent, based ontotal film thickness (more preferably, from about 5 to about 15percent). Preferably, the film comprises a crosslinked polymer network.

Preferably, the film has an impact strength, as measured by ASTM 3763(hereby incorporated by reference thereto, in its entirety), of at leastabout 60 Newtons (N); more preferably, from about 60 to about 500 N; yetmore preferably, from about 70 to about 500 N; yet still morepreferably, from about 80 to about 500 N; more preferably, from about 90to about 500 N; more preferably, from about 100 to about 500 N; morepreferably, from about 110 to about 500 N; more preferably, from about120 to about 500 N.

Preferably, the film has an impact strength (peak load) at 190° F. (88°C.), as measured by ASTM 3763 conducted at 88° C., of at least about 10pounds (i.e., lbf or poundforce); more preferably, from about 10 toabout 150 pounds; more preferably, from about 12 to about 100 pounds;more preferably, from about 14 to about 75 pounds; more preferably, fromabout 16 to about 60 pounds; more preferably, from about 18 to about 50pounds; and more preferably, from about 20 to about 40 pounds.

As a second aspect, the present invention pertains to a bag made fromthe film according to the first aspect of the present invention.Preferably, the bag is made from a preferred film according to the firstaspect of the present invention. Preferably, the bag is produced bysealing the first layer to itself, whereby the first layer is an insidebag layer and the fourth layer is an outside bag layer. Preferably, thebag is made from a preferred film according to the first aspect of thepresent invention. The bag can be an end-seal bag, a side-seal bag, anL-seal bag (i.e., sealed across the bottom and along one side, with anopen top), or a pouch (i.e., sealed on three sides, with an open top).

As a third aspect, the present invention is directed to a process forpackaging a product, comprising the steps of: (A) placing a firstproduct into a flexible, heat-shrinkable bag which is in accordance withthe second aspect of the present invention; (B) repeating the placingstep with a second product and a second bag, whereby a second baggedproduct results; (C) stacking at least the first and second baggedproducts so that an excess bag length of each of the bagged products arewithin a sealing distance of a means for heat-sealing; and (D)heat-sealing the inside layer of first bag to itself in the regionbetween the open end of the first bag and the first product, and theinside layer of the second bag to itself in the region between the openend of the second bag and the second product, so that the first productis completely sealed within the first bag and the second product iscompletely sealed with the second bag. The sealing is carried out at atemperature so that the resulting packaged products can be freelyseparated from one another without layer delamination. The bag has anopen top so that prior to sealing, both the first bagged product and thesecond bagged product have excess bag length. During sealing of eachbag, the first layer is sealed to itself, as the first layer is theinside layer in both the first bag and the second bag. Likewise, thefourth layer is the outside layer of the first bag and the second bag.The process can be carried out in a continuous, single, dual, or rotarychamber vacuum packaging machine. Preferably, from 2 to 5 baggedproducts are stacked on top of one another during heat-sealing.Preferably, the process utilizes a preferred bag in accordance with thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of a first preferred process formaking a multilayer film in accordance with the present invention.

FIG. 2 illustrates a schematic view of a second preferred process formaking a multilayer film in accordance with the present invention.

FIG. 3 illustrates a schematic of an end-seal bag in accordance with thepresent invention, in lay-flat view.

FIG. 4 illustrates a schematic of a side-seal bag in accordance with thepresent invention, in lay-flat view.

FIG. 5 illustrates a schematic of a stack sealing process.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the phrase “free shrink” refers to the percentdimensional change in a 10 cm×10 cm specimen of film, when subjected toselected heat (i.e., at a certain temperature), with the quantitativedetermination being carried out according to ASTM D 2732, as set forthin the 1990 Annual Book of ASTM Standards, Vol. 08.02, pp.368–371, whichis hereby incorporated, in its entirety, by reference thereto.

The multilayer film according to the present invention preferably has atotal free shrink of at least 40 percent at 185° F. “Total free shrink”is determined by summing the percent free shrink in the machinedirection with the percentage of free shrink in the transversedirection. For example, a film which exhibits, at 185° F., 50% freeshrink in the transverse direction and 40% free shrink in the machinedirection, has a “total free shrink” at 185° F. of 90%. It should benoted, however, the film does not have to have shrinkage in bothdirections. Unless specified otherwise, the phrase “free shrink”, asused herein, refers to total free shrink. Preferably, the multilayerfilm of the present invention has a total free shrink at 185° F. of from40 to about 170 percent; more preferably, from about 50 to about 150percent; more preferably, from about 60 to about 130 percent; morepreferably, from about 65 to about 110 percent; more preferably, fromabout 70 to about 100 percent; and, more preferably, from about 75 toabout 95 percent.

Preferably, the film has a gloss (as measured by ASTM D2457) of at least50% (preferably, from about 50 to about 90%), more preferably 60%(preferably, from about 60 to about 90%), more preferably 65%(preferably, from about 65% to about 90%), more preferably 70%(preferably, from about 70% to about 90%), and more preferably 75%(preferably, from about 75% to about 90%).

The percent haze of film is determined by subjecting the film toanalysis by ASTM D1003. This method is described in detail in 1990Annual Book of ASTM Standards, Section 8, Vol. 08.01, ASTM D 1003,“Standard Test Method for Haze and Luminous Transmittance of TransparentPlastics”, pp. 358–363, which is hereby incorporated by referencethereto, in its entirety. Haze results were obtained using an XL 211HAZEGARD (™) SYSTEM, obtained from the Gardner/Neotec InstrumentDivision, of Silver Spring, Maryland. This instrument requires a minimumsample size of about 1 square inch. Preferably, the film of the instantinvention has a haze of less than 10 percent; more preferably, from 0 toabout 9 percent; more preferably, from about 1 to about 8 percent; morepreferably, from about 2 to about 7 percent; 0.5 still more preferably,from about 2 to about 6; yet still more preferably, from about 2 toabout 5 percent; and yet still more preferably, from about 2 to about 4percent.

Haze is a measurement of the transmitted light scattered more than 2.5°from the axis of the incident light. It is measured with a meter similarto a total light transmission meter, with the exception that it containsa light trap to absorb light scattered less than 2.5° and regulartransmitted light. It is common to measure the total transmitted lightfirst by defeating the light trap and then setting the meter to 100.Then the light trap is allowed to absorb the light scattered less than2.5° (plus regular transmitted light), and haze is read as a percentageof total transmitted light. Note that the denominator here is totaltransmitted light (I_(s)+I_(r)), not incident light (I_(i)), as in themeasurement of total transmitted light.

The measurement of optical properties of plastic films used inpackaging, including the measurement of total transmission, haze,clarity (i.e., total transmission) and gloss, is discussed in detail inPike, LeRoy, “Optical Properties of Packaging Materials”, Journal ofPlastic film & sheeting, Vol. 9, No. 3, pp. 173–180 (July 1993), whichis hereby incorporated by reference thereto, in its entirety.

As used herein, the term “film” is used in a generic sense to includeplastic web, regardless of whether it is film or sheet. Preferably,films of and used in the present invention have a thickness of 0.25 mmor less. As used herein, the term “package” refers to packagingmaterials used in the packaging of a product.

As used herein, the phrases “seal layer”, “sealing layer”, “heat seallayer”, and “sealant layer”, refer to an outer layer, or layers,involved in the sealing of the film to itself, another layer of the sameor another film, and/or another article which is not a film. Although itshould also be recognized that in general, up to the outer 3 mils of afilm can be involved in the sealing of the film to itself or anotherlayer, the phrase “seal layer,” and the like, refer herein only to theouter film layer(s) which is to be heat-sealed to itself, another film,etc. Any inner film layers which contribute to the sealing performanceof the film are herein designated as “seal-assist” layers. With respectto packages having only fin-type seals, as opposed to lap-type seals,the phrase “sealant layer” generally refers to the inside layer of apackage, the inside layer being an outer film layer which frequentlyalso serves as a food contact layer in the packaging of foods. However,in a multilayer film, the composition of the other layers (within 3 milsof the inside surface) can also affect sealability and seal strength.

In general, sealant layers employed in the packaging art have includedthe genus of thermoplastic polymer, including thermoplastic polyolefinpolyamide, polyester, polyvinyl chloride, and ionomer. Preferredpolymers for the sealant layer include homogeneous ethylene/alpha-olefincopolymer, ethylene/vinyl acetate copolymer, and ionomer.

As used herein, the term “seal” refers to any seal of a first region ofa film surface to a second region of a film surface, wherein the seal isformed by heating the regions to at least their respective sealinitiation temperatures. The heating can be performed by any one or moreof a wide variety of manners, such as using a heated bar, hot wire, hotair, infrared radiation, ultrasonic sealing, etc. Heat-sealing is theprocess of joining two or more thermoplastic films or sheets by heatingareas in contact with each other to the temperature at which fusionoccurs, usually aided by pressure. Heat-sealing is inclusive of thermalsealing, melt-bead sealing, impulse sealing, dielectric sealing, andultrasonic sealing.

As used herein, the term “barrier”, and the phrase “barrier layer”, asapplied to films and/or layers, is used with reference to the ability ofa film or layer to serve as a barrier to one or more gases. In thepackaging art, oxygen (i.e., gaseous O₂) barrier layers have, ingeneral, included, for example, ethylene/vinyl alcohol copolymer(polymerized ethylene vinyl alcohol), polyvinylidene chloride (PVDC),polyalkylene carbonate, polyamide, polyethylene naphthalate, polyester,polyacrylonitrile, etc., as known to those of skill in the art. However,in the present invention the O₂-barrier layer preferably compriseseither EVOH or polyvinylidene chloride, the PVDC comprising a thermalstabilizer (i.e., HCl scavenger, e.g., epoxidized soybean oil) and alubricating processing aid, which, for example, comprises one or moreacrylates.

As used herein, the phrases “abuse layer”, as well as the phrase“puncture-resistant layer”, refer to any film layer which serves toresist abrasion, puncture, and other potential causes of reduction ofpackage integrity, as well as potential causes of reduction of packageappearance quality. As used herein, the phrase “skin layer” refers to anoutside layer of a multilayer film in packaging a product, this skinlayer being subject to abuse.

As used herein, the term “core”, and the phrase “core layer”, as appliedto multilayer films, refer to any internal layer which preferably has afunction other than serving as an adhesive or compatibilizer foradhering two layers to one another. Usually, the core layer or layersprovide the multilayer film with a desired level of strength, i.e.,modulus, and/or optics, and/or added abuse resistance, and/or specificimpermeability.

As used herein, the phrase “tie layer” refers to any internal layerhaving the primary purpose of adhering two layers to one another. In onepreferred embodiment, tie layers can comprise any polymer having a polargroup grafted thereon, so that the polymer is capable of covalentbonding to polar polymers such as polyamide and ethylene/vinyl alcoholcopolymer. Preferred polymers for use in tie layers include, but are notrestricted to, ethylene/unsaturated acid copolymer, ethylene/unsaturatedester copolymer, anhydride-grafted polyolefin, polyurethane, andmixtures thereof.

As used herein, the phrase “bulk layer” refers to any layer of a filmwhich is present for the purpose of increasing the abuse-resistance,toughness, modulus, orientability, etc., of a multilayer film. Bulklayers generally comprise polymers which are inexpensive relative toother polymers in the film.

As used herein, the phrases “food-contact layer” and “meat-contactlayer”, refer to a layer of a multilayer film which is in direct contactwith the food/meat in the package comprising the film. Thefood-contact/meat-contact layer is an outer layer of the multilayerfilm, in the sense that the food-contact/meat-contact layer is in directcontact with the meat product within the package. Thefood-contact/meat-contact layer is an inside layer in the sense thatwith respect to the packaged food product/meat product, thefood-contact/meat-contact layer is the inside layer (i.e., innermostlayer) of the package, this inside layer being in direct contact withthe food/meat.

As used herein, the phrase “food-contact surface” and “meat-contactsurface” refers to an outer surface of a food-contact layer/meat-contactlayer, this outer surface being in direct contact with the food/meatwithin the package.

As used herein, the phrase “thickness uniformity” refers to percentvalue obtained by measuring the maximum and minimum thickness of thefilm and applying these numbers to the following formula:

${{Thickness}\mspace{14mu}{Uniformity}\mspace{14mu}(\%)} = {100 - {\frac{{{film}\mspace{14mu}{thickness}_{(\max)}} - {{film}\mspace{14mu}{thickness}_{(\min)}}}{{film}\mspace{14mu}{thickness}_{(\max)}} \times 100.}}$The maximum and minimum thicknesses are determined by taking a total of10 thickness measurements at regular distance intervals along theentirety of the transverse direction of a film sample, recording thehighest and lowest thickness values as the maximum and minimum thicknessvalues, respectively, and computing the thickness uniformity (a percentvalue) using the formula above. A thickness uniformity of 100%represents a film of absolute thickness uniformity, i.e., no measurabledifferences in thickness; in contrast, a film in which the filmthickness (min) is measured at 45% of the film thickness (max) has athickness uniformity of only 45%.

As used herein, “EVOH” refers to ethylene/vinyl alcohol copolymer. EVOHincludes saponified or hydrolyzed ethylene/vinyl acetate copolymers, andrefers to a vinyl alcohol copolymer having an ethylene comonomer, andprepared by, for example, hydrolysis of vinyl acetate copolymers, or bychemical reactions with polyvinyl alcohol. The degree of hydrolysis ispreferably at least 50%, and more preferably, at least 85%. Preferably,the EVOH comprises from about 28 to about 48 mole % ethylene, morepreferably, from about 32 to about 44 mole % ethylene, and even morepreferably, from about 38 to about 44 mole % ethylene.

As used herein, the term “lamination”, the term “laminate”, and thephrase “laminated film”, refer to the process, and resulting product,made by bonding together two or more layers of film or other materials.Lamination can be accomplished by joining layers with adhesives, joiningwith heat and pressure, and even spread coating and extrusion coating.The term laminate is also inclusive of coextruded multilayer filmscomprising one or more tie layers.

As used herein, the term “oriented” refers to a polymer-containingmaterial which has been stretched at an elevated temperature (theorientation temperature), followed by being “set” in the stretchedconfiguration by cooling the material while substantially retaining thestretched dimensions. Upon subsequently heating unrestrained,unannealed, oriented polymer-containing material to its orientationtemperature, heat shrinkage is produced almost to the originalunstretched, i.e., pre-oriented dimensions. More particularly, the term“oriented”, as used herein, refers to oriented films, wherein theorientation can be produced in one or more of a variety of manners.

As used herein, the phrase “orientation ratio” refers to themultiplication product of the extent to which the plastic film materialis expanded in several directions, usually two directions perpendicularto one another. Expansion in the machine direction is herein referred toas “drawing”, whereas expansion in the transverse direction is hereinreferred to as “stretching”. For films extruded through an annular die,stretching is usually obtained by “blowing” the film to produce abubble. For such films, drawing is usually obtained by passing the filmthrough two sets of powered nip rolls, with the downstream set having ahigher surface speed than the upstream set, with the resulting drawratio being the surface speed of the downstream set of nip rolls dividedby the surface speed of the upstream set of nip rolls. The degree oforientation is also referred to as the orientation ratio, or sometimesas the “racking ratio”.

As used herein, the phrase “machine direction”, herein abbreviated “MD”,refers to a direction “along the length” of the film, i.e., in thedirection of the film as the film is formed during extrusion and/orcoating.

As used herein, the phrase “transverse direction”, herein abbreviated“TD”, refers to a direction across the film, perpendicular to themachine or longitudinal direction.

As used herein, the phrases “heat-shrinkable,” “heat-shrink,” and thelike, refer to the tendency of a film, generally an oriented film, toshrink upon the application of heat, i.e., to contract upon beingheated, such that the size (area) of the film decreases while the filmis in an unrestrained state. Likewise, the tension of a heat-shrinkablefilm increases upon the application of heat if the film is restrainedfrom shrinking. As a corollary, the phrase “heat-contracted” refers to aheat-shrinkable film, or a portion thereof, which has been exposed toheat such that the film or portion thereof is in a heat-shrunken state,i.e., reduced in size (unrestrained) or under increased tension(restrained). Preferably, the heat-shrinkable film has a total freeshrink (i.e., machine direction plus transverse direction), as measuredby ASTM D 2732, of at least as 5 percent at 185° C., more preferably atleast 7 percent, still more preferably, at least 10 percent, and, yetstill more preferably, at least 20 percent.

The multilayer films of the invention can be annealed or heat-set toreduce the free shrink either slightly, substantially or completely.

As used herein, the term “monomer” refers to a relatively simplecompound, usually containing carbon and of low molecular weight, whichcan react to form a polymer by combining with itself or with othersimilar molecules or compounds.

As used herein, the term “comonomer” refers to a monomer which iscopolymerized with at least one different monomer in a copolymerizationreaction, the result of which is a copolymer.

As used herein, the term “polymer” refers to the product of apolymerization reaction, and is inclusive of homopolymers, copolymers,terpolymers, etc. In general, the layers of a film can consistessentially of a single polymer, or can have still additional polymerstogether therewith, i.e., blended therewith.

As used herein, the term “homopolymer” is used with reference to apolymer resulting from the polymerization of a single monomer, i.e., apolymer consisting essentially of a single type of mer, i.e., repeatingunit.

As used herein, the term “copolymer” refers to polymers formed by thepolymerization reaction of at least two different monomers. For example,the term “copolymer” includes the copolymerization reaction product ofethylene and an alpha-olefin, such as 1-hexene. However, the term“copolymer” is also inclusive of, for example, the copolymerization of amixture of ethylene, propylene, 1-hexene, and 1-octene. The term“copolymer” is also inclusive of random copolymers, block copolymers,and graft copolymers.

As used herein, the term “polymerization” is inclusive ofhomopolymerizations, copolymerizations, terpolymerizations, etc., andincludes all types of copolymerizations such as random, graft block,etc. In general, the polymers in the films used in accordance with thepresent invention can be prepared in accordance with any suitablepolymerization process, including slurry polymerization, gas phasepolymerization, and high pressure polymerization processes.

As used herein, the term “copolymerization” refers to the simultaneouspolymerization of two or more monomers to result in a copolymer. As usedherein, a copolymer identified in terms of a plurality of monomers,e.g., “propylene/ethylene copolymer”, refers to a copolymer in whicheither monomer may copolymerize in a higher weight or molar percent thanthe other monomer or monomers. However, the first listed monomerpreferably polymerizes in a higher weight percent than the second listedmonomer, and, for copolymers which are terpolymers, quadripolymers,etc., preferably the first monomer copolymerizes in a higher weightpercent than the second monomer, and the second monomer copolymerizes ina higher weight percent than the third monomer, etc.

For addition polymers, copolymers are identified, i.e., named, in termsof the monomers from which the copolymers are produced. For example, thephrase “propylene/ethylene copolymer” refers to a copolymer produced bythe copolymerization of both propylene and ethylene, with or withoutadditional comonomer(s). A copolymer comprises recurring “mers” derivedfrom the monomers from which the copolymer is produced, e.g., apropylene/ethylene copolymer comprises propylene mer units and ethylenemer units.

As used herein, terminology employing a “/” with respect to the chemicalidentity of a copolymer (e.g., “an ethylene/alpha-olefin copolymer”),identifies the comonomers which are copolymerized to produce thecopolymer. As used herein, “ethylene alpha-olefin copolymer” is theequivalent of “ethylene/alpha-olefin copolymer.”

As used herein, the phrase “heterogeneous polymer” refers topolymerization reaction products of relatively wide variation inmolecular weight and relatively wide variation in compositiondistribution, i.e., typical polymers prepared, for example, usingconventional Ziegler-Natta catalysts. Heterogeneous polymers are usefulin various layers of the film used in the present invention. Althoughthere are a few exceptions (such as TAFMER (™) linear homogeneousethylene/alpha-olefin copolymers produced by Mitsui PetrochemicalCorporation, using Ziegler-Natta catalysts), heterogeneous polymerstypically contain a relatively wide variety of chain lengths andcomonomer percentages.

As used herein, the phrase “homogeneous polymer” refers topolymerization reaction products of relatively narrow molecular weightdistribution and relatively narrow composition distribution. Homogeneouspolymers are useful in various layers of the multilayer film used in thepresent invention. Homogeneous polymers are structurally different fromheterogeneous polymers, in that homogeneous polymers exhibit arelatively even sequencing of comonomers within a chain, a mirroring ofsequence distribution in all chains, and a similarity of length of allchains, i.e., a narrower molecular weight distribution. Furthermore,homogeneous polymers are typically prepared using metallocene, or othersingle-site type catalysis, rather than using Ziegler Natta catalysts.

More particularly, homogeneous ethylene/alpha-olefin copolymers may becharacterized by one or more methods known to those of skill in the art,such as molecular weight distribution (M_(w)/M_(n)), compositiondistribution breadth index (CDBI), and narrow melting point range andsingle melt point behavior. The molecular weight distribution(M_(w)/M_(n)), also known as polydispersity, may be determined by gelpermeation chromatography. The homogeneous ethylene/alpha-olefincopolymers useful in this invention generally have (M_(w)/M_(n)) of lessthan 2.7; preferably from about 1.9 to about 2.5; more preferably, fromabout 1.9 to about 2.3. The composition distribution breadth index(CDBI) of such homogeneous ethylene/alpha-olefin copolymers willgenerally be greater than about 70 percent. The CDBI is defined as theweight percent of the copolymer molecules having a comonomer contentwithin 50 percent (i.e., plus or minus 50%) of the median total molarcomonomer content. The CDBI of linear polyethylene, which does notcontain a comonomer, is defined to be 100%. The Composition DistributionBreadth Index (CDBI) is determined via the technique of TemperatureRising Elution Fractionation (TREF). CDBI determination clearlydistinguishes the homogeneous copolymers used in the present invention(narrow composition distribution as assessed by CDBI values generallyabove 70%) from VLDPEs available commercially which generally have abroad composition distribution as assessed by CDBI values generally lessthan 55%. The CDBI of a copolymer is readily calculated from dataobtained from techniques known in the art, such as, for example,temperature rising elution fractionation as described, for example, inWild et. al., J. Poly. Sci. Poly. Phys. Ed., Vol. 20, p.441 (1982).Preferably, the homogeneous ethylene/alpha-olefin copolymers have a CDBIgreater than about 70%, i.e., a CDBI of from about 70% to about 99%. Ingeneral, the homogeneous ethylene/alpha-olefin copolymers in themultilayer films of the present invention also exhibit a relativelynarrow melting point range, in comparison with “heterogeneouscopolymers”, i.e., polymers having a CDBI of less than 55%. Preferably,the homogeneous ethylene/alpha-olefin copolymers exhibit an essentiallysingular melting point characteristic, with a peak melting point(T_(m)), as determined by Differential Scanning Colorimetry (DSC), offrom about 60° C. to about 105° C. Preferably the homogeneous copolymerhas a DSC peak T_(m) of from about 80° C. to about 100° C. As usedherein, the phrase “essentially single melting point” means that atleast about 80%, by weight, of the material corresponds to a singleT_(m) peak at a temperature within the range of from about 60° C. toabout 105° C., and essentially no substantial fraction of the materialhas a peak melting point in excess of about 115° C., as determined byDSC analysis. DSC measurements are made on a Perkin Elmer System 7Thermal Analysis System. Melting information reported are second meltingdata, i.e., the sample is heated at a programmed rate of 10° C./min. toa temperature below its critical range. The sample is then reheated (2ndmelting) at a programmed rate of 10° C./min. The presence of highermelting peaks is detrimental to film properties such as haze, andcompromises the chances for meaningful reduction in the seal initiationtemperature of the final film.

A homogeneous ethylene/alpha-olefin copolymer can, in general, beprepared by the copolymerization of ethylene and any one or morealpha-olefins. Preferably, the alpha-olefin is a C₃–C₂₀alpha-monoolefin, more preferably, a C₄–C₁₂ alpha-monoolefin, still morepreferably, a C₄–C₈ alpha-monoolefin. Still more preferably, thealpha-olefin comprises at least one member selected from the groupconsisting of butene-1, hexene-1, and octene-1, i.e., 1-butene,1-hexene, and 1-octene, respectively. Most preferably, the alpha-olefincomprises octene-1, and/or a blend of hexene-1 and butene-1.

Processes for preparing and using homogeneous polymers are disclosed inU.S. Pat. Nos. 5,206,075, 5,241,031, and PCT International ApplicationWO 93/03093, each of which is hereby incorporated by reference thereto,in its entirety. Further details regarding the production and use ofhomogeneous ethylene/alpha-olefin copolymers are disclosed in PCTInternational Publication Number WO 90/03414, and PCT InternationalPublication Number WO 93/03093, both of which designate Exxon ChemicalPatents, Inc. as the Applicant, and both of which are herebyincorporated by reference thereto, in their respective entireties.

Still another genus of homogeneous ethylene/alpha-olefin copolymers isdisclosed in U.S. Pat. No. 5,272,236, to LAI, et. al., and U.S. Pat. No.5,278,272, to LAI, et. al., both of which are hereby incorporated byreference thereto, in their respective entireties.

As used herein, the term “polyolefin” refers to any polymerized olefin,which can be linear, branched, cyclic, aliphatic, aromatic, substituted,or unsubstituted. More specifically, included in the term polyolefin arehomopolymers of olefin, copolymers of olefin, copolymers of an olefinand an non-olefinic comonomer copolymerizable with the olefin, such asvinyl monomers, modified polymers thereof, and the like. Specificexamples include polyethylene homopolymer, polypropylene homopolymer,polybutene, ethylene/alpha-olefin copolymer, propylene/alpha-olefincopolymer, butene/alpha-olefin copolymer, ethylene/unsaturated estercopolymer, ethylene/unsaturated acid copolymer, (especially ethylacrylate copolymer, ethylene/butyl acrylate copolymer, ethylene/methylacrylate copolymer, ethylene/acrylic acid copolymer,ethylene/methacrylic acid copolymer), modified polyolefin resin, ionomerresin, polymethylpentene, etc. Modified polyolefin resin is inclusive ofmodified polymer prepared by copolymerizing the homopolymer of theolefin or copolymer thereof with an unsaturated carboxylic acid, e.g.,maleic acid, fumaric acid or the like, or a derivative thereof such asthe anhydride, ester or metal salt or the like. It could also beobtained by incorporating into the olefin homopolymer or copolymer, anunsaturated carboxylic acid, e.g., maleic acid, fumaric acid or thelike, or a derivative thereof such as the anhydride, ester or metal saltor the like.

As used herein, terms identifying polymers, such as “polyamide”,“polyester”, “polyurethane”, etc. are inclusive of not only polymerscomprising repeating units derived from monomers known to polymerize toform a polymer of the named type, but are also inclusive of comonomers,derivatives, etc. which can copolymerize with monomers known topolymerize to produce the named polymer. For example, the term“polyamide” encompasses both polymers comprising repeating units derivedfrom monomers, such as caprolactam, which polymerize to form apolyamide, as well as copolymers derived from the copolymerization ofcaprolactam with a comonomer which when polymerized alone does notresult in the formation of a polyamide. Furthermore, terms identifyingpolymers are also inclusive of mixtures, blends, etc. of such polymerswith other polymers of a different type.

As used herein, the phrase “modified polymer”, as well as more specificphrases such as “modified ethylene/vinyl acetate copolymer”, and“modified polyolefin” refer to such polymers having an anhydridefunctionality, as defined immediately above, grafted thereon and/orcopolymerized therewith and/or blended therewith. Preferably, suchmodified polymers have the anhydride functionality grafted on orpolymerized therewith, as opposed to merely blended therewith.

As used herein, the phrase “anhydride-containing polymer” and“anhydride-modified polymer”, refer to one or more of the following: (1)polymers obtained by copolymerizing an anhydride-containing monomer witha second, different monomer, and (2) anhydride grafted copolymers, and(3) a mixture of a polymer and an anhydride-containing compound.

As used herein, the phrase “ethylene alpha-olefin copolymer”, and“ethylene/alpha-olefin copolymer”, refer to such heterogeneous materialsas linear low density polyethylene (LLDPE), and very low and ultra lowdensity polyethylene (VLDPE and ULDPE); and homogeneous polymers such asmetallocene-catalyzed EXACT (™) linear homogeneous ethylene/alpha olefincopolymer resins obtainable from the Exxon Chemical Company, of Baytown,Texas, and TAFMER (™) linear homogeneous ethylene/alpha-olefin copolymerresins obtainable from the Mitsui Petrochemical Corporation. All thesematerials generally include copolymers of ethylene with one or morecomonomers selected from C₄ to C₁₀ alpha-olefin such as butene-1 (i.e.,1-butene), hexene-1, octene-1, etc. in which the molecules of thecopolymers comprise long chains with relatively few side chain branchesor cross-linked structures. This molecular structure is to be contrastedwith conventional low or medium density polyethylenes which are morehighly branched than their respective counterparts. The heterogeneousethylene/alpha-olefin commonly known as LLDPE has a density usually inthe range of from about 0.91 grams per cubic centimeter to about 0.94grams per cubic centimeter. Other ethylene/alpha-olefin copolymers, suchas the long chain branched homogeneous ethylene/alpha-olefin copolymersavailable from The Dow Chemical Company, known as AFFINITY (™) resins,are also included as another type of homogeneous ethylene/alpha-olefincopolymer useful in the present invention.

In general, the ethylene/alpha-olefin copolymer comprises a copolymerresulting from the copolymerization of from about 80 to about 99 weightpercent ethylene and from 1 to about 20 weight percent alpha-olefin.Preferably, the ethylene/alpha-olefin copolymer comprises a copolymerresulting from the copolymerization of from about 85 to about 95 weightpercent ethylene and from about 5 to about 15 weight percentalpha-olefin.

As used herein, the phrases “inner layer” and “internal layer” refer toany layer, of a multilayer film, having both of its principal surfacesdirectly adhered to another layer of the film.

As used herein, the phrase “outer layer” refers to any layer of filmhaving less than two of its principal surfaces directly adhered toanother layer of the film. The phrase is inclusive of monolayer andmultilayer films. In multilayer films, there are two outer layers, eachof which has a principal surface adhered to only one other layer of themultilayer film. In monolayer films, there is only one layer, which, ofcourse, is an outer layer in that neither of its two principal surfacesare adhered to another layer of the film.

As used herein, the phrase “inside layer” refers to the outer layer, ofa multilayer film packaging a product, which is closest to the product,relative to the other layers of the multilayer film. “Inside layer” alsois used with reference to the innermost layer of a plurality ofconcentrically arranged layers simultaneously coextruded through anannular die.

As used herein, the phrase “outside layer” refers to the outer layer, ofa multilayer film packaging a product, which is furthest from theproduct relative to the other layers of the multilayer film. The phrase“outside layer” also is used with reference to the outermost layer of aplurality of concentrically arranged layers coextruded through anannular die.

As used herein, the term “adhered” is inclusive of films which aredirectly adhered to one another using a heat-seal or other means, aswell as films which are adhered to one another using an adhesive whichis between the two films. As used herein, the phrase “directly adhered”,as applied to layers, is defined as adhesion of the subject layer to theobject layer, without a tie layer, adhesive, or other layertherebetween. In contrast, as used herein, the word “between”, asapplied to a layer expressed as being between two other specifiedlayers, includes both direct adherence of the subject layer between tothe two other layers it is between, as well as including a lack ofdirect adherence to either or both of the two other layers the subjectlayer is between, i.e., one or more additional layers can be imposedbetween the subject layer and one or more of the layers the subjectlayer is between.

As used herein, the term “extrusion” is used with reference to theprocess of forming continuous shapes by forcing a molten plasticmaterial through a die, followed by cooling or chemical hardening.Immediately prior to extrusion through the die, the relativelyhigh-viscosity polymeric material is fed into a rotating screw ofvariable pitch, i.e., an extruder, which forces the polymeric materialthrough the die.

As used herein, the term “coextrusion” refers to the process ofextruding two or more materials through a single die with two or moreorifices arranged so that the extrudates merge and weld together into alaminar structure before chilling, i.e., quenching. Coextrusion can beemployed in film blowing, free film extrusion, and extrusion coatingprocesses.

At least a portion of the multilayer film of the present invention ispreferably irradiated to induce crosslinking. In the irradiationprocess, the film is subjected to one or more energetic radiationtreatment, such corona discharge, plasma, flame, ultraviolet, X-ray,gamma ray, beta ray, and high energy electron treatment, each of whichinduce cross-linking between molecules of the irradiated material. Theirradiation of polymeric films is disclosed in U.S. Pat. No. 4,064,296,to BORNSTEIN, et. al., which is hereby incorporated in its entirety, byreference thereto. BORNSTEIN, et. al. discloses the use of ionizingradiation for crosslinking the polymer present in the film.

To produce crosslinking, a suitable radiation dosage of high energyelectrons is employed, preferably using an electron accelerator, with adosage level being determined by standard dosimetry methods. Otheraccelerators such as a Van de Graaf or resonating transformer may beused. The radiation is not limited to electrons from an acceleratorsince any ionizing radiation may be used. The ionizing radiation can beused to crosslink the polymers in the film. Preferably, the film isirradiated at a level of from about 30 kGy to about 207 kGy, morepreferably from about 30 kGy to about 140 kGy. As can be seen from thedescriptions of preferred films for use in the present invention, themost preferred amount of radiation is dependent upon the film and itsend use.

As used herein, the phrases “corona treatment” and “corona dischargetreatment” refer to subjecting the surfaces of thermoplastic materials,such as polyolefins, to corona discharge, i.e., the ionization of a gassuch as air in close proximity to a film surface, the ionizationinitiated by a high voltage passed through a nearby electrode, andcausing oxidation and other changes to the film surface, such as surfaceroughness.

Corona treatment of polymeric materials is disclosed in U.S. Pat. No.4,120,716, to BONET, issued Oct. 17, 1978, herein incorporated in itsentirety by reference thereto. BONET discloses improved adherencecharacteristics of the surface of polyethylene by corona treatment, tooxidize the polyethylene surface. U.S. Pat. No. 4,879,430, to HOFFMAN,also hereby incorporated in its entirety by reference thereto, disclosesthe use of corona discharge for the treatment of plastic webs for use inmeat cook-in packaging, with the corona treatment of the inside surfaceof the web to increase the adhesion of the meat to the adhesion of themeat to the proteinaceous material. The films of this invention can becorona-treated in a preferred embodiment.

Preferably, the film according to the present invention comprises atotal of from 4 to 20 layers; more preferably, from 4 to 12 layers; andstill more preferably, from 5 to 9 layers. The multilayer film of thepresent invention can have any total thickness desired, so long as thefilm provides the desired properties for the particular packagingoperation in which the film is used, e.g. optics, modulus, sealstrength, etc.

The multilayer film according to the present invention comprises 4layers. The first layer comprises at least one polyolefin; morepreferably, at least one member selected from the group consisting ofpolyethylene homopolymer, polyethylene copolymer, polypropylenehomopolymer, polypropylene copolymer, polybutene homopolymer, polybutenecopolymer; still more preferably, at least one member selected from thegroup consisting of ethylene/alpha-olefin copolymer,ethylene/unsaturated ester copolymer, and ethylene/unsaturated acidcopolymer. More preferably, the first layer comprises a homogeneousethylene/alpha-olefin copolymer. Preferably, the ethylene/alpha-olefincopolymer has a density of less than 0.930 g/cc (preferably, from about0.86 to less than 0.930, more preferably, from about 0.880 to less than0.930), more preferably, less than 0.920, more preferably, less than0.915, more preferably, less than 0.910, more preferably less than0.905, more preferably, less than 0.903, more preferably, less than0.900, more preferably, less than 0.898. In general, for the films ofthe invention, the lower the density of the polyolefin in the seallayer, the better the sealability and resistance to burn-through.

Preferably, the polyolefin in the first layer has a melting point lessthan about 125° C. (more preferably, from about 50° C. to less than 125°C.; still more preferably, from about 70° C. to less than about 125°C.); more preferably, less than 120° C.; more preferably, less than 115°C.; more preferably, less than 110° C.; more preferably, less than 108°C.; more preferably, less than 105° C.; more preferably, less than 100°C.; more preferably, less than 97° C.; more preferably less than 95° C.;more preferably, less than 93° C., more preferably, less than 90° C.

The first layer can further comprise additional polymer(s) in an amountof from about 5 to about 80 percent, based on layer weight, morepreferably from about 10 to about 40% and even more preferably, fromabout 10 to about 20%. Preferred additional polymers include at leastone member selected from the group consisting of polyolefin,polystyrene, polyamide, polyester, polymerized ethylene vinyl alcohol,polyvinylidene chloride, polyether, polyurethane, polycarbonate, andstarch-containing polymer; more preferably, at least one member selectedfrom the group consisting of ethylene/alpha-olefin copolymer,propylene/alpha-olefin copolymer, butene/alpha-olefin copolymer,ethylene/unsaturated ester copolymer, and ethylene/unsaturated acidcopolymer.

In one preferred embodiment, the first layer comprises a blend of ahomogeneous ethylene-alpha olefin copolymer with a heterogeneousethylene alpha-olefin copolymer. Preferably, the homogeneousethylene-alpha olefin copolymer has a density less than about 0.910 g/cc(preferably, from about 0.86 to less than about 0.910; more preferably,from about 0.88 to less than about 0.910) and the heterogeneousethylene-alpha olefin copolymer has a density greater than about 0.910.

Preferably, the polyolefin of the first layer has a melt index of fromabout 0.3 to about 50 g/10 min; more preferably from about 1 to about20; still more preferably from about 2 to about 10, even more preferablyfrom about 3 to about 8; and, still more preferably from about 4 toabout 6 (as measured by ASTM D1238, which is hereby incorporated, in itsentirety, by reference thereto).

Preferably, the first layer has a thickness of from about 0.1 to about 4mils; more preferably, from about 0.2 to about 1 mil; and, still morepreferably, from about 0.3 to about 0.8 mils. Preferably, the thicknessof the first layer is from about 1 to about 60 percent of the totalthickness of the multilayer film; more preferably, about 5 to about 50%;more preferably, about 10 to about 40%; more preferably, about 15 toabout 35%; and even more preferably, about 15 to about 30%. Preferably,the thickness of the first layer is at least 50% of the thickness of thethird layer; more preferably, at least 75% of the thickness of the thirdlayer; more preferably, at least 100% of the thickness of the thirdlayer; more preferably, at least 125% of the thickness of the thirdlayer; and, even more preferably, at least 150% of the thickness of thethird layer.

In the first layer, the use of a polyolefin with a low melting point(preferably, less than 120° C. {more preferably, from about 70° C. toless than 120° C.; still more preferably, from about 80° C. to less than120° C.}; more preferably, less than about 110° C., e.g., homogeneousethylene/alpha olefin copolymers) provides the advantage of ease ofsealability of the film, and resistance to burn-through of the film. Itis believed that these advantages are realized because the use of alower melting polyolefin in the seal layer permits the use of a lowersealing temperature and widens the sealing window, thereby reducing thetendency to burn through. Furthermore, the use of a lower sealingtemperature enhances the use of this film for applications involvingstack/overlap sealing. In addition, the use of a polyolefin with a lowmelting point, in combination with an inner layer comprising apolyester, provides the article of manufacture made therefrom with highseal strength. The use of the preferred polyolefins as described in thefirst layer also enables the film of the invention to be oriented moreeasily and provide the multilayer film with high free shrink andexcellent optics.

The second layer comprises at least one member selected from the groupconsisting of polyolefin, polystyrene and polyurethane; more preferably,at least one member selected from the group consisting of polyethylenehomopolymer, polyethylene copolymer, polypropylene homopolymer,polypropylene copolymer, polybutene homopolymer, polybutene copolymer;even more preferably, at least one member selected from the groupconsisting of ethylene/alpha-olefin copolymer, ethylene/unsaturatedester copolymer, and ethylene/unsaturated acid copolymer. Still morepreferably, the second layer comprises an ethylene-vinyl acetatecopolymer. A preferred ethylene/vinyl acetate copolymer comprises fromabout 3 to about 28% vinyl acetate comonomer, more preferably, fromabout 5 to about 20% vinyl acetate comonomer, even more preferably, fromabout 8 to about 18% vinyl acetate copolymer and even yet still morepreferably, from about 12 to about 18% vinyl acetate comonomer.

The second layer can further comprise additional polymer in an amount offrom about 5 to about 80% based on layer weight, more preferably fromabout 10 to about 40% and even more preferably, from about 10 to about20%. Preferred additional polymers include at least one member selectedfrom the group consisting of polyolefin, polystyrene, polyamide,polyester, polymerized ethylene vinyl alcohol, polyvinylidene chloride,polyether, polyurethane, polycarbonate, and starch-containing polymer;more preferably, at least one member selected from the group consistingof ethylene/alpha-olefin copolymer, propylene/alpha-olefin copolymer,butene/alpha-olefin copolymer, ethylene/unsaturated ester copolymer, andethylene/unsaturated acid copolymer. The second layer facilitates theorientation process used to produce the film of the invention and alsoprovides the film with excellent free shrink, impact resistance andoptics. Optionally, the second layer could also serve as a tie layer.

The polymer of the second layer preferably has a melt index of fromabout 0.3 to about 50, more preferably from about 1 to about 20, stillmore preferably from about 1 to about 10, even more preferably fromabout 1 to about 5, and, still more preferably from about 1 to about 3.

Preferably, the second layer has a thickness of from about 0.05 to about4 mils; more preferably, from about 0.1 to about 1 mil; and, still morepreferably, from about 0.2 to about 0.7 mils. Preferably, the thicknessof the second layer is from about 1 to about 50 percent, based on totalfilm thickness; more preferably, from about 5 to about 50 percent; morepreferably, from about 8 to about 50 percent; more preferably, fromabout 10 to about 45 percent; more preferably, from about 13 to about 40percent; more preferably, from about 15 to about 35 percent; morepreferably, from about 17 to about 25 percent; and more preferably, fromabout 20 to about 25 percent. Preferably, the thickness of the secondlayer is at least 50% of the thickness of the third layer, morepreferably, at least 75% of the thickness of the third layer, morepreferably, at least 100% of the thickness of the third layer, morepreferably, at least 125% of the thickness of the third layer and evenmore preferably, at least 150% of the thickness of the third layer.

The third layer comprises a polyester. While the polyester utilized inthe third layer could be a homopolymer or a copolymer, preferably thepolyester is a copolyester. Preferably, the polyester comprises fromabout 70 to about 95 mole percent terephthalate mer units; morepreferably, from about 80 to about 95 mole percent terephthalate merunits; and still more preferably, from about 85 to about 90 mole percentterephthalate mer units.

Preferably, the polyester has a melting point of from about 130° C. toabout 260° C.; more preferably, from about 150° C. to about 250° C.;even more preferably, from about 170° C. to about 250° C.; still morepreferably, from about 180° C. to about 240° C.; still more preferably,from about 190° C. to about 240° C.; still more preferably, from about200° C. to about 240° C.; and, yet still more preferably, from about210° C. to about 235° C. In another preferred embodiment, the polyesterin the third layer is an amorphous polyester (i.e., has no crystallinityor relatively low crystallinity). Examples of suitable polyestersinclude PET homopolymer, PET copolymer, PEN homopolymer, and PENcopolymer.

The use of a polyester in the third layer, in conjunction with an outerlayer comprising at least one member selected from the group consistingof polyester, polyamide, polypropylene and polyurethane, providescertain unexpected results when compared to the use of polyolefins orpolyamides with high melting points (i.e., melting points greater than160° C.) in the third layer. For example, it has been discovered that inthe multilayer film of the invention, the use of an inner layercomprising a polyester, in conjunction with an outer layer comprisingpolyester: increases the impact strength of the film at roomtemperature; increases the impact strength of the film at elevatedtemperatures, relative to the impact strength of films which arepredominantly polyolefinic; provides superior film optics; and providessuperior grease-resistance. Furthermore, it has also been discoveredthat the films of this invention: are easier to orient; and can beoriented to a higher degree, when compared to other films, especiallythose comprising an inner polyamide layer having a high melting point.The ease of orientability and wider orientation window provided thefilms of this invention also result in a more stable orientationprocess. Additionally, the films of this invention can also be orientedat a lower temperature than films comprising an inner layer comprising apolyamide with a high melting point, e.g. polyamide 6. While thecomposition of the films of this invention also provide higher freeshrink, the process of orientation at a lower temperature also enhancesthe free shrink of the multilayer films of this invention. Furthermore,the films of this invention are also relatively free of optical defects(such as die-lines), versus films comprising an inner layer comprising apolyamide with a high melting point, i.e., greater than 160° C. (e.g.,polyamide 6). Furthermore, the use of a polyester facilitates theincorporation and orientation of thicker layers of polyester, therebyproviding superior impact strength (as compared to films comprising aninner layer comprising a high melting polyamide).

Preferably, the third layer has a thickness of from about 5 to about 40percent, based on the total thickness of the multilayer film; morepreferably, from about 7 to about 30 percent; more preferably, fromabout 10 to about 28 percent; more preferably, from about 12 to 26percent; and more preferably, from about 18 to about 25 percent. If thethickness of the third layer is less than about 5% of the totalthickness of the multilayer film, the film exhibits aless-than-preferred impact strength, toughness, and puncture resistance.On the other hand, if the thickness of the third layer is greater thanabout 40% of the total thickness of the multilayer film, the film has ashrink and clarity which are less-than-preferred; moreover, the tape,from which the film is made, becomes more difficult to orient.Preferably, the third layer has a thickness of from about 0.05 to about2 mils; more preferably, from about 0.1 to about 1 mil; still morepreferably, from about 0.2 to about 0.8 mil; yet still more preferably,from about 0.2 to about 0.4 mil; and, even yet still more preferably,from about 0.2 to about 0.3 mil.

The fourth layer comprises at least one member selected from the groupconsisting of polyester, polyamide, polypropylene and polyurethane.Preferably, the polyester has a melting point of from about 130° C. toabout 260° C.; more preferably, from about 150° C. to about 250° C.;even more preferably from about 170° C. to about 250° C.; still morepreferably, from about 180° C. to about 240° C.; still more preferably,from about 190° C. to about 240° C.; still more preferably, from about200° C. to about 240° C.; and yet still more preferably, from about 210°C. to about 235° C. For applications requiring stack sealability, highermelting point polyester is preferred. Preferably, the higher meltingpoint polyester comprises polyester having a terephthalic acid mercontent of at least 75 mole percent; more preferably, at least 80 molepercent; more preferably, at least 85 mole percent; and even morepreferably, at least 90 mole percent. In another preferred embodiment,the polyester in the fourth layer is an amorphous polyester, morepreferably, an amorphous copolyester.

While the polyester utilized in the fourth layer could be a homopolymeror a copolymer, preferably, the polyester is a copolyester. Preferably,the polyester comprises from about 70 to about 95 mole percentterephthalate mer units; more preferably, from about 80 to about 95 molepercent terephthalate mer units; and still more preferably, from about85 to about 90 mole percent terephthalate mer units. For applicationsrequiring overlap sealability, polyester having a higher mole percent ofterephthalate mer units is preferred. Examples of suitable polyesterinclude PET homopolymer, PET copolymer, PEN homopolymer, and PENcopolymer.

In another preferred embodiment, the fourth layer comprises polyamide.Preferably, the polyamide comprises at least one member selected fromthe group consisting of polyamide 6, polyamide 9, polyamide 10,polyamide 11, polyamide 12, polyamide 66, polyamide 610, polyamide 612,polyamide 6I, polyamide 6T, polyamide 69, and copolymers thereof.Preferably, the polyamide has a melting point of from about 130° C. toabout 235° C.; more preferably, from about 150° C. to about 235° C.;more preferably, from about 170° C. to about 235° C.; more preferably,from about 180° C. to about 235° C.; and more preferably from about 195°C. to about 235° C. In another preferred embodiment, the fourth layercomprises a polypropylene; preferably a polypropylene copolymer; morepreferably, a polypropylene copolymer comprising at least 70% propylenemer; more preferably, from about 70 to about 99% propylene mer; morepreferably, from about 80 to about 99% propylene mer; more preferably,from about 85 to about 99% propylene mer; more preferably, from about 90to about 99% propylene mer; and more preferably, from about 94 to about99% propylene mer. Preferably, the polypropylene has a melting point ofat least 120° C., more preferably, from about 120° C. to about 160° C.;more preferably, from about 130° C. to about 150° C.; and even morepreferably, from about 135° C. to about 145° C. In another preferredembodiment, the fourth layer comprises a polyurethane, preferably havinga melting point of at least 120° C.

Preferably, the fourth layer has a thickness of from about 0.05 to about4 mils; more preferably, from about 0.1 to about 1 mil; and, still morepreferably, from about 0.2 to about 0.8 mils. Preferably, the thicknessof the fifth layer is from about 1 to about 30 percent of the totalthickness of the multilayer film; more preferably, from about 4 to about20 percent; still more preferably, from about 4 to about 16 percent;still more preferably, from about 5 to about 15 percent; and, still morepreferably, from about 7 to about 12 percent.

Preferably, the film according to the present invention furthercomprises a fifth layer which has O₂-barrier characteristics.Preferably, the fifth layer has a thickness of from about 0.05 to about2 mils; more preferably, from about 0.05 to about 0.5 mil; yet stillmore preferably, from about 0.1 to about 0.3 mil; and even yet stillmore preferably, from about 0.12 to about 0.17 mils. Preferably, thefifth layer comprises at least one member selected from the groupconsisting of EVOH, PVDC, polyalkylene carbonate, polyamide, andpolyethylene naphthalate; more preferably, EVOH having about 44 molepercent ethylene mer. Preferably, the thickness of the fifth layer isfrom about 1 to about 25 percent of the total thickness of themultilayer film; more preferably, about 3 to about 18 percent; and,still more preferably, from about 5 to about 15 percent.

Preferably, the film further comprises a sixth layer which has acomposition which is similar to or identical with the composition of thethird layer, as described above. Optionally or additionally, the sixthlayer comprises a polyamide. Preferably, the polyamide comprises atleast one member selected from the group consisting of polyamide 6,polyamide 9, polyamide 10, polyamide 11, polyamide 12, polyamide 66,polyamide 610, polyamide 612, polyamide 6I, polyamide 6T, polyamide 69,and copolymers thereof.

Preferably, the film further comprises a seventh layer, and morepreferably, an eighth layer. The seventh layer and the eighth layerpreferably serve as tie layers. The thickness and composition of tielayers used in the films of this invention are as known to those ofskill in the art.

Preferably, the third layer and the fourth layer are not adjacent oneanother. Even more preferably, the multilayer film of this invention hasat least one layer between the third layer and the fourth layer, thislayer comprising a polymer having a lower modulus than the polyamide orpolyester in the third or fourth layers. Preferably, the polymerreferred to above, which has a lower modulus than the polyamide orpolyester in the third or fourth layer, comprises at least one memberselected from the group consisting of polyolefin, polystyrene,polyurethane, EVOH, polyalkylene carbonate, and PVDC. In one preferredexample, the layer having a lower modulus than the polyamide orpolyester can also serve as a tie layer, or as an O₂-barrier layer.

Preferably, the film comprises polyester in an amount of at least 10percent polyester, based on the total weight of film; more preferably,from about 5 percent to about 60 percent; more preferably, from about 10percent to about 50 percent; more preferably, from about 15 percent toabout 40 percent; more preferably, from about 20 percent to about 40percent; and even more preferably, from about 25 percent to about 35percent.

Preferably, the film exhibits a modulus of at least 40,000 psi (morepreferably, from about 40,000 to about 250,000 psi); more preferably, atleast about 50,000; more preferably, at least about 60,000; morepreferably, at least about 70,000; more preferably at least about80,000; more preferably, at least about 90,000; more preferably, atleast about 100,000; more preferably, at least about 110,000; and, morepreferably, at least about 120,000 psi. Modulus is measured inaccordance with ASTM D 882, the entirety of which is hereby incorporatedby reference thereto.

Preferably, the film exhibits a shrink tension in at least one directionof at least 100 psi, more preferably 175 psi; still more preferably,from about 175 to about 500 psi; still more preferably, from about 200to about 500 psi; more preferably, from about 225 to about 500 psi; morepreferably, from about 250 to about 500 psi; more preferably, from about275 to about 500 psi; more preferably, from about 300 to about 500 psi;and more preferably, from about 325 to about 500 psi. Shrink tension ismeasured in accordance with ASTM D 2838, the entirety of which is herebyincorporated by reference thereto.

Various combinations of layers can be used in the formation of themultilayer films according to the invention. Only 4-, 5-, and 6-layerpreferred embodiments are provided here as illustrations. The multilayerfilms of the invention can also comprise more layers. Thus,modifications and variations may be utilized without departing from theprinciples and scope of the invention, as those skilled in the art willreadily understand. Given below are some examples of preferredcombinations in which the alphabetical symbols used designate thefollowing resin layers: “A” represents a layer comprising polyolefin,preferably as described in the description of the first layer; “B”represents a layer comprising at least one member selected from thegroup consisting of polyolefin, polystyrene, and polyurethane,preferably as described in the description of the second layer; “C”represents a layer comprising polyester, preferably as described in thedescription of the third layer; and, “D” represents a layer comprisingat least one member selected from the group consisting of polyester,polyamide, polypropylene and polyurethane, preferably as described inthe description of the fourth layer.

Various preferred multilayer films can be prepared in accordance withthe film and process of the present invention, as follows: A/B/C/D,A/C/B/D, A/B/C/E/D, A/B/E/C/D, A/C/B/E/D, A/C/E/B/D, A/E/B/C/D,A/E/C/B/D, A/C/B/C/D, A/B/C/B/D, A/B/C/E/B/D, A/B/C/E/C/D, A/B/E/C/B/D,A/C/E/C/B/D, A/B/C/B/B′/D, A/C/B/B′/B″/D, A/C/B/C/B/D, A/C/B/E/B′/D. Inany one of these multilayer structures, a plurality of layers (A), (B),and (C) may be formed of the same or different modified compositions.

Preferably, the film is produced by casting an annular tape which isthereafter oriented at least 2.7:1 in at least 1 direction; morepreferably, from about 2.7:1 to about 10:1 in at least one direction;still more preferably, at least 2.8:1; still more preferably, at least2.9:1, yet still more preferably, at least 3.0:1; even yet still morepreferably, at least 3.1:1; yet still more preferably, at least 3.2:1yet still more preferably, at least 3.3:1; yet still more preferably, atleast 3.2:4; yet still more preferably, at least 3.5:1; yet still morepreferably, at least 3.6:1; and, yet still more preferably, at least3.7:1. Films comprising polyamide with a melting point greater than 160°C. in the inner layer are difficult to orient at a ratio of about 2.7 to1 in the transverse direction; above 3.0 orientation becomes even moredifficult. However, the films of this invention can be easily orientedat least 3:1 in at least one direction.

FIG. 1 illustrates a schematic view of a first preferred process formaking films according to the present invention. As illustrated in FIG.1, solid polymer beads (not illustrated) are fed to a plurality ofextruders 28 (for simplicity, only one extruder is illustrated). Insideextruders 28, the polymer beads are forwarded, melted, and degassed,following which the resulting bubble-free melt is forwarded into diehead 30, and extruded through an annular die, resulting in tubing 32which is preferably about 10 to 20 mils thick.

After cooling or quenching by water spray from cooling ring 34, tubing32 is collapsed by pinch rolls 36, and is thereafter fed throughirradiation vault 38 surrounded by shielding 40, where tubing 32 isirradiated with high energy electrons (i.e., ionizing radiation) fromiron core transformer accelerator 42. Tubing 32 is guided throughirradiation vault 38 on rolls 44. Preferably, tubing 32 is irradiated toa level of from about 40 kGy to about 120 kGy.

After irradiation, irradiated tubing 46 is directed through pinch rolls48, following which irradiated tubing 46 is slightly inflated, resultingin trapped bubble 50. However, at trapped bubble 50, the tubing is notsignificantly drawn longitudinally, as the surface speed of nip rolls 52are about the same speed as nip rolls 48. Furthermore, irradiated tubing46 is inflated only enough to provide a substantially circular tubingwithout significant transverse orientation, i.e., without stretching.

Slightly inflated, irradiated tubing 46 is passed through vacuum chamber54, and thereafter forwarded through coating die 56. Annular coatingstream 58 is melt extruded from coating die 56 and coated onto slightlyinflated, irradiated tube 50, to form two-ply tubular film 60. Coatingstream 58 preferably comprises an O₂-barrier layer, which does not passthrough the ionizing radiation. Further details of the above-describedcoating step are generally as set forth in U.S. Pat. No. 4,278,738, toBRAX et. al., which is hereby incorporated by reference thereto, in itsentirety.

After irradiation and coating, two-ply tubing film 60 is wound up ontowindup roll 62. Thereafter, windup roll 62 is removed and installed asunwind roll 64, on a second stage in the process of making the tubingfilm as ultimately desired. Two-ply tubular film 60, from unwind roll64, is unwound and passed over guide roll 66, after which two-plytubular film 60 passes into hot water bath tank 68 containing hot water70. The now collapsed, irradiated, coated tubular film 60 is immersed inhot water 70 (preferably, having temperature of about 185° F. to 210°F.) for a period of from about 10 to about 100 seconds, i.e., for a timeperiod in order to bring the film up to the desired temperature forbiaxial orientation.

Thereafter, irradiated tubular film 60 is directed through nip rolls 72,and bubble 74 is blown, thereby transversely stretching tubular film 60.Furthermore, while being blown, i.e., transversely stretched, nip rolls76 draw tubular film 60 in the longitudinal direction, as nip rolls 76have a surface speed higher than the surface speed of nip rolls 72. As aresult of the transverse stretching and longitudinal drawing,irradiated, coated biaxially-oriented blown tubing film 78 is produced,this blown tubing preferably having been both stretched in a ratio offrom about 1:1.5 to about 1:6, and drawn at a ratio of from about 1:1.5to about 1:6; more preferably, the stretching and drawing are eachperformed a ratio of from about 1:2 to about 1:4. The result is abiaxial orientation of from about 1:2.25 to about 1:36, more preferably,from about 1:4 to about 1:16. While bubble 74 is maintained betweenpinch rolls 72 and 76, blown tubing 78 is collapsed by rollers 80, andthereafter conveyed through pinch rolls 76 and across guide roll 82, andthen rolled onto wind-up roll 84. Idler roll 86 assures a good wind-up.

FIG. 2 illustrates a schematic of a second preferred process for makinga film in accordance with the present invention. In FIG. 2, solidpolymer beads (not illustrated) are fed to a plurality of extruders (forsimplicity, only extruder 88 is illustrated). Inside extruders 88, thepolymer beads are forwarded, melted, and degassed, following which theresulting bubble-free melt is forwarded into die head 90, and extrudedthrough an annular die, resulting in tubing tape 92 which is preferablyfrom about 10 to 20 mils thick, and preferably has a lay-flat width offrom about 2 to 10 inches.

After cooling or quenching by water spray from cooling ring 94, tubingtape 92 is collapsed by pinch rolls 96, and is thereafter fed throughirradiation vault 98 surrounded by shielding 100, where tubing 92 isirradiated with high energy electrons (i.e., ionizing radiation) fromiron core transformer accelerator 102. Tubing 92 is guided throughirradiation vault 98 on rolls 104. Preferably, tubing 92 is irradiatedto a level of from about 40 to about 120 kGy, resulting in irradiatedtubing 106, which is then passed over guide roll 116, after whichirradiated tubing 106 is passed into and through hot water bath tank 118containing hot water 120. Irradiated tubing 106 is immersed in hot water120 (preferably having a temperature of about 185 to about 210° F.) fora period of about 10 to about 100 seconds, i.e., for a time period longenough to bring the film up to the desired temperature for biaxialorientation. Thereafter, the resulting hot, irradiated tubing 122 isdirected through nip rolls 124, and bubble 126 is blown, therebytransversely stretching hot, irradiated tubular tubing 122 so that anoriented film tube 128 is formed. Furthermore, while being blown, i.e.,transversely stretched, nip rolls 130 have a surface speed higher thanthe surface speed of nip rolls 124, thereby resulting in longitudinalorientation. As a result of the transverse stretching and longitudinaldrawing, oriented film tube 128 is produced, this blown tubingpreferably having been both stretched at a ratio of from about 1:1.5 toabout 1:6, and drawn at a ratio of from about 1:1.5 to about 1:6. Morepreferably, the stretching and drawing are each performed at a ratio offrom about 1:2 to about 1:4. The result is a biaxial orientation of fromabout 1:2.25 to about 1:36, more preferably, from about 1:4 to about1:16. While bubble 126 is maintained between pinch rolls 124 and 130,oriented film tube 128 is collapsed by rollers 132, and thereafterconveyed through pinch rolls 130 and across guide roll 134, and thenrolled onto wind-up roll 136. Idler roll 138 assures a good wind-up.This process can be carried out continuously in a single operation, orintermittently, e.g., as a two-stage process, in which the extruded,irradiated tape is wound up after irradiation, and, after a period ofstorage, unwound and subjected to heating and orienting in order toarrive at oriented film tubing 128.

FIG. 5 is a schematic illustration of a stack sealing process in whichvacuum chamber 172 holds first product 174 which has been placed infirst bag 176 and second product 178 which has been placed in second bag180, with the resulting bagged products being stacked on top of oneanother, with excess bag length of each of first bag 176 and second bag178 positioned on top of one another and within sealing distance of ameans for sealing 182, ready for subsequent evacuation and sealing.

The invention is illustrated by the following examples, which areprovided for the purpose of representation, and are not to be construedas limiting the scope of the invention. Unless stated otherwise, allpercentages, parts, etc. are by weight.

Film No. 1

A preferred eight-layer, heat-shrinkable multilayer film according tothe present invention was produced in a manner as illustrated in FIG. 2,described above. The composition of this film, referred to herein asFilm No. 1, is described below and provided in Table 1. Film No. 1 is anexample of the multilayer film of the current invention. The first layerwas an outer layer and which served as a seal layer, inside bag layer,and product-contact layer; (2) the second layer comprised polyolefin;(3) the third layer comprised a polyester; (4) the fourth layer was anouter layer and comprised a polyester; (5) the fifth layer served as anO₂-barrier layer; (6) the sixth layer comprised polyester; (7) theseventh layer served as a tie layer; and (8) the eighth layer served asa tie layer.

TABLE I (Characteristics of Film No. 1) Layer Layer Designation LayerChemical Identity Thickness (mils) first 80% homogeneous ethylene/alpha-0.43 olefin copolymer; and 20% LLDPE second EVA 0.36 seventh modifiedEMA 0.13 third Polyester #1 0.18 fifth EVOH 0.13 sixth Polyester #1 0.18eighth modified EMA 0.16 fourth Polyester #2 0.13

The layer arrangement was in the order of Table 1, above. The resinsidentified in the table were as follows:

Homogeneous ethylene/alpha-olefin copolymer was AFFINITY® DPL 1280 longchain branched substantially linear single site catalyzedethylene/octene copolymer containing 13 weight percent octene mer andhaving a density of 0.900 g/cc and a melt index of 6.0 grams/min. Thisresin was obtained from The Dow Chemical Company, of Midland, Mich.

LLDPE was ESCORENE® LL3003.32 linear low density polyethylene having 90weight percent ethylene mer 10 weight percent hexene mer, having adensity of 0.9175 g/cc and a melt index of 3.2 g/min. This resin wasobtained from Exxon Chemical Americas, of Houston, Tex.

EVA was PE 5269T ethylene/vinyl acetate copolymer having a vinyl acetatecontent of 6.5 percent, a melt index of 0.5 g/min, and a density of0.9315 g/cc. This resin was obtained from the Chevron Chemical Company,of Houston, Tex.

Modified EMA was BYNEL® 2174 anhydride grafted ethylene/methyl acrylatecopolymer having a melt index of 2.8 and a density of 0.931 g/cc. Thisresin was obtained from the Dupont Company, of Wilmington, Del.

Polyester #1 was EASTAR® 6763 PETG polyethylene terephthalatecopolyester modified with cyclohexanedimethanol, which had an intrinsicviscosity of 0.74 at 220° C., and which was obtained from EastmanChemical Company of Kingsport, Tennessee. This is essentially anamorphous polyester.

EVOH was EVAL® LC-E105A ethylene/vinyl alcohol copolymer, and contained44 mole percent ethylene and had a melting point of 166.5° C. This resinwas obtained from Eval Company of America, of Lisle, Ill.

Polyester #2 was SELAR® PT 8307 modified polyester copolymer resin, andhad a melting point of 220° C. (as measured by differential scanningcalorimetry, according to ASTM 3410), a density of 1.33 g/cc (asmeasured by differential scanning calorimetry, according to ASTM D1505),and an intrinsic viscosity of 0.71 (measured by the DuPont method). Thisresin was obtained from the Chevron Chemical Company, of Houston, Tex.

Film No. 1 had a total thickness of 1.7 mils, a peak load impactstrength of 117.3 N (at 73° F.), a peak load impact strength of 18.8 lbs(at 190° F.), a total free shrink at 185° F. of 81 percent, a gloss of77 percent, and a haze of 5.0 percent. Film No. 1 also had a modulus (at73° F.) of 102,000 psi, significantly higher than that of polyolefinicfilms. In summary, Film No. 1 had excellent impact strength, high freeshrink, and high optics in terms of significantly low haze and highgloss.

Film No. 1 was especially suited to making a bag with the first layer asthe inside layer and the eighth layer as the outside layer. Such bagsmade from Film No. 1 can be stacked on top of one another andsimultaneously sealed by a single sealing means, and thereafter furtherprocessed, without sticking to one another.

The extrusion and orientation to produce Film No. 1 was surprisinglyeasy, considering the fact that it had two layers comprising polyester,had good bubble stability and excellent thickness uniformity (greaterthan 70%). Film No. 1 also had an unexpectedly high impact strength,especially considering its thickness of only 1.7 mils. This was quiteunexpected and is believed to be due to the incorporation of tworelatively thin layers of polyester as opposed to a single thickerlayer. Based on this unexpected result, it is now believed that theincorporation of two discrete layers, one comprising a polyester and theother comprising a polyester, polyamide and/or polyurethane, willprovide the films of this invention with the required modulus and also,surprisingly, enhance the impact strength, especially at elevatedtemperatures. Thus, we get a synergistic affect by separating theselayers from one another, preferably through the incorporation of a lowermodulus polymer. It was equally surprising that despite using an innerlayer comprising a polyester and an outer layer comprising a polyester,the film of this invention had relatively high free shrink. As a resultof this surprising discovery, it is now believed that by separating outthe two polyester layers in the film of this invention, the polyolefiniclayers were allowed to control or decide (determine) the free shrink ofthe multilayer film. Taking this analogy one step further, the inventorsalso believe that the same principle could be used for films comprisingan inner layer comprising polyester and an outer layer comprisingpolyamide or polyurethane. While downgauging of films significantlyreduces the abuse-resistance and impact strength of films, because ofthe surprising advantages described above, the multilayer films of theinvention can be made in downgauged versions (i.e., thinner), whilestill providing abuse-resistance and impact strength comparable/superiorto other thicker films.

Film No. 2

Film No. 2, another preferred eight-layer, heat-shrinkable multilayerfilm according to the present invention, can be produced in the samegeneral manner that Film No. 1 was produced. Film No. 2 is of thechemical composition as described in Table 2 and is also characterizedby good impact strength, high gloss and high degree of free shrink.

TABLE 2 (Characteristics of Film No. 2) Layer Layer Designation LayerChemical Identity Thickness (mils) first 80% homogeneous ethylene/alpha-0.43 olefin copolymer; and 20% LLDPE second EVA 0.36 seventh modifiedEMA 0.13 third Polyester #2 0.18 fifth EVOH 0.13 sixth Polyester #2 0.18eighth modified EMA 0.16 fourth Polyester #2 0.13

The layer arrangement is in the order of Table 2, above. The resinsspecified in Table 2 are as identified above, i.e., below Table 1.

Comparative Film No. 3

Comparative Film No. 3 has been used for the packaging of fresh red meatand is illustrated for comparative purposes. Film No. 3 had the chemicalcomposition as set forth in Table 3, below. Film No. 3 was about 2.0mils thick and had a peak load impact strength of about 120 Newtons (at73° F.), a peak load impact strength of about 14 lbs (at 190° F.), atotal free shrink at 185° F. of about 80 percent, gloss of about 81%, ahaze of about 5%, and a modulus (at 73° F.) of about 32, 600 psi.

TABLE 3 (Characteristics of Film No. 3) Layer Layer Designation LayerChemical Identity Thickness (mils) first 80% homogeneous ethylene/alpha-0.44 olefin copolymer; and 20% LLDPE second EVA #2 0.90 third PVDC 0.18fourth 90% EVA #3 and 10% HDPE #1 0.48

The layer arrangement was in the order of Table 3, above. The resinsidentified in Table 3 were as follows: Homogeneous ethylene/alpha-olefincopolymer and LLDPE are as identified above, i.e., below Table 1.

EVA#2 was ESCORENE® LD-720.92 ethylene/vinyl acetate copolymer having avinyl acetate content of 18 percent, a melt index of 1.5 g/min, and adensity of 0.940 g/cc. This resin was obtained from Exxon ChemicalAmericas, of Houston, Tex.

EVA#3 was ESCORENE® LD-318.92 ethylene/vinyl acetate copolymer having avinyl acetate content of 9 percent, a melt index of 2 g/min, and adensity of 0.930 g/cc. This resin was obtained from Exxon ChemicalAmericas, of Houston, Tex.

PVDC was a PVDC/MA copolymer obtained from The Dow Chemical Company ofMidland, Mich.

HDPE#1 was Fortiflex® T60-500-199 high density polyethylene resin with adensity of 0.961 g/cc and a melt index of 6.2 g/min. This resin wasobtained from Solvay Polymers of Deer Park, Tex.

The film of Example 1 possessed significantly higher impact strength (atan elevated temperature, e.g., 190° F.) than the Comparative Film No. 3,and also significantly higher modulus. However, as can be seen, opticaland shrink properties are not compromised by the incorporation of thelayers comprising polyesters.

FIG. 3 is a schematic of a preferred end seal bag 140, in a lay-flatposition, this bag being in accord with the present invention. Bag 140comprises bag film 142, top edge 144 defining an open top, first bagside edge 146, second bag side edge 148, bottom edge 150, and end seal152.

FIG. 4 illustrates side-seal bag 160, an alternative bag according tothe present invention. Side seal bag 160 is comprised of bag film 162,top edge 164 defining an open top, bottom edge 166, first side seal 168,and second side seal 170.

Although the bag according to the present invention can be used in thepackaging of any product, the bag of the present invention is especiallyadvantageous for the packaging of food products, especially processedmeat products and fresh meat products. Among the types of meat which canbe packaged in the films and packages according to the present inventionare poultry, pork, beef, lamb, goat, horse, and fish. Preferably, thebag of the present invention is used in the packaging of boneless meatproducts, such as boneless beef, pork, poultry, lamb, and fish products.

In another preferred embodiment, the multilayer film of the inventioncan be used as a bag or as a tubular casing, preferably a shirrablecasing. Preferably, the casing is used for the packaging of foodproducts, especially processed meat products and fresh red meatproducts. Among the types of meat which can be packaged in the films andpackages according to the present invention are poultry, pork, beef,sausage, lamb, goat, horse, and fish. Preferably, the casing of thepresent invention is used in the packaging of pork, poultry, beef, andsausage products.

The polymer components used to fabricate multilayer films according tothe present invention may also contain appropriate amounts of otheradditives normally included in such compositions. These include slipagents such as talc, antioxidants, fillers, dyes, pigments and dyes,radiation stabilizers, antistatic agents, elastomers, and the likeadditives known to those of skill in the art of packaging films.

All ranges within all of the above-disclosed ranges are expresslyincluded within this specification. Moreover, layers which are adjacentor directly adhered to one another are preferably of differing chemicalcomposition, especially differing polymeric composition. All referenceto ASTM tests are to the most recent, currently approved and publishedversion of the ASTM test identified, as of the priority filing date ofthis application.

Although the present invention has been described in connection with thepreferred embodiments, it is to be understood that modifications andvariations may be utilized without departing from the principles andscope of the invention, as those skilled in the art will readilyunderstand. Accordingly, such modifications may be practiced within thescope of the following claims.

1. A process for packaging a product, comprising the steps of: (A)placing a first product into a flexible, heat-shrinkable bag, the baghaving an open top, whereby a first bagged product having excess baglength results, and wherein the bag comprises a heat-shrinkablemultilayer film comprising: (1) a first layer, which is an inside baglayer, and which comprises polyolefin; (2) a second layer comprising atleast one member selected from the group consisting of polyolefin,polystyrene, and polyurethane; (3) a third layer comprising at least onemember selected from the group consisting of amorphous polyester andpolyester having a melting point of from about 130° C. to about 260° C.;and (4) a fourth layer, which is an outside bag layer, the fourth layercomprising at least one member selected from the group consisting ofpolyester, polyamide, polypropylene and polyurethane; and wherein thebag is produced by sealing the first layer to itself, whereby the firstlayer is an inside bag layer and the fourth layer is an outside baglayer; (B) repeating the placing step with a second product and a secondbag, whereby a second bagged product results; (C) stacking at least thefirst and second bagged products so that the excess bag length of eachof the bagged products are on top of one another and within a sealingdistance of a means for heat-sealing; (D) heat-sealing the inside layerof first bag to itself in the region between the open end of the firstbag and the product, and the inside layer of the second bag to itself inthe region between the open end of the second bag and the product, sothat the first product is completely sealed within the first bag and thesecond product is completely sealed with the second bag, the sealingbeing carried out at a temperature so that the resulting packagedproducts can be freely separated from one another without layerdelamination.
 2. The process according to claim 1, wherein the secondlayer has a thickness of from about 5 to about 50%, based on thethickness of the heat-shrinkable multilayer film.
 3. The processaccording to claim 1, wherein the heat-shrinkable film further comprisesa fifth layer which serves as an O₂-barrier layer, the fifth layercomprising at least one member selected from the group consisting ofEVOH, PVDC, polyalkylene carbonate, polyamide, and polyethylenenaphthalate.
 4. The process according to claim 1, wherein the process iscarried out in a rotary chamber vacuum machine.
 5. The process accordingto claim 4, wherein 2 bagged products are stacked on top of one anotherduring heat-sealing.
 6. The process according to claim 1, wherein from 2to 5 bagged products are stacked on top of one another duringheat-sealing.
 7. The process according to claim 1, further comprisingevacuating the first and second bags after they are stacked but beforethey are sealed.
 8. The process according to claim 1, wherein the firstbag and the second bag are made from films having the same multilayerstructure and composition.
 9. The process according to claim 1, whereinthe film has a total free shrink, at 185° F., of from about 40 to 170percent.
 10. The process according to claim 1, wherein the third layercomprises an amorphous polyester and the fourth layer comprises at leastone member selected from the group consisting of amorphous polyester andpolyester having a melting point of from about 130° C. to about 260° C.11. The process according to claim 1, wherein the fourth layer comprisesat least one member selected from the group consisting of amorphouspolyamide and polyamide having a melting point of from about 130° C. toabout 260° C.
 12. The process according to claim 1, wherein the fourthlayer comprises a polyester having from about 70 to 95 mole percentterephthalate mer units.
 13. The process according to claim 1, whereinthe film has a gloss of at least 50 percent, as measured against thefourth layer by ASTM D2457.
 14. The process according to claim 1,wherein the film has a total thickness of from about 1 to about 5 mils.15. The process according to claim 14, wherein the film has a totalthickness of from about 1.5 to about 3 mils.
 16. The process accordingto claim 1, wherein the film further comprises a fifth layer whichserves as an O₂-barrier layer and which is between the third layer andthe fourth layer, the fifth layer comprising at least one memberselected from the group consisting of EVOH, PVDC, polyalkylenecarbonate, polyamide, and polyethylene naphthalate.
 17. The processaccording to claim 16, further comprising a sixth layer which comprisesat least one member selected from the group consisting of polyester andpolyamide, the sixth layer being between the fourth layer and the fifthlayer.
 18. The process according to claim 16, wherein the first layercomprises ethylene/alpha-olefin copolymer; the second layer comprisesethylene/vinyl acetate copolymer; the third layer comprises polyethyleneterephthalate; the fourth layer comprises polyethylene terephthalate;and, the fifth layer comprises EVOH.
 19. The process according to claim16, wherein, based on total film thickness, the first layer has athickness of from about 1 to 60 percent, the second layer has athickness of from about 1 to 50 percent, the third layer has a thicknessof from about 5 to 40 percent, the fourth layer has a thickness of fromabout 1 to 40 percent, and, the fifth layer has a thickness of fromabout 1 to 20 percent.
 20. The process according to claim 1, wherein thefirst layer comprises a blend of homogeneous ethylene/alpha-olefincopolymer and heterogeneous ethylene/alpha-olefin copolymer.
 21. Theprocess according to claim 1, wherein the film comprises a crosslinkedpolymer network.
 22. The process according to claim 1, wherein the filmhas a total free shrink, at 185° F., of from about 60 to 150 percent; animpact strength of at least 60 Newtons, as measured by ASTM D3763; agloss of at least 50 percent, as measured by ASTM D2457; and a haze ofless than 10%, as measured by ASTM D1003.
 23. The process according toclaim 1, wherein the first layer, consists essentially of at least onemember selected from the group consisting of ethylene homopolymer,ethylene copolymer, propylene homopolymer, propylene copolymer, butenehomopolymer, butene copolymer, polystyrene, polyamide, polyester,polyurethane, and starch-containing polymer.
 24. The process accordingto claim 23, wherein the first layer consists essentially of at leastone member selected from the group comprising ethylene/alpha-olefincopolymer, ethylene/unsaturated ester copolymer, ethylene/unsaturatedacid copolymer.
 25. The process according to claim 24, wherein the firstlayer consists essentially of homogeneous ethylene/alpha-olefincopolymer.
 26. The process according to claim 1, wherein: the thirdlayer of the multilayer film comprises at least one member selected fromthe group consisting of amorphous polyester and polyester having amelting point of from about 130° C. to less than 190° C.; and the fourthlayer of the multilayer film comprises at least one member selected fromthe group consisting of polyester having a melting point of at least190° C., polyamide, and polyurethane.
 27. The process according to claim26, wherein the multilayer film further comprises a fifth layer whichserves as an O₂-barrier layer and which is between the third layer andthe fourth layer, the fifth layer comprising EVOH.
 28. The processaccording to claim 27, wherein the multilayer film further comprises asixth layer which comprises at least one member selected from the groupconsisting of amorphous polyester and polyester having a melting pointof from about 130° C. to less than 190° C., the sixth layer beingbetween the fourth layer and the fifth layer.
 29. The process accordingto claim 27, wherein the fourth layer comprises the polyester having amelting point of at least 190° C.
 30. The process according to claim 28,wherein the fourth layer comprises the polyester having a melting pointof at least 190° C.